Whole House Generator Sizing: What Size Do I Need?

Start whole house generator sizing with the backup goal—not house size

Whole house generator sizing should start with the loads you actually need during an outage—not square footage alone. Before comparing generator kilowatts, decide which circuits and equipment should work during an outage. An essentials plan might prioritize refrigeration, lights, communications, a sump or well pump, and a heating-system blower. A broader plan may include air conditioning, cooking, laundry, or more than one large motor. The home's square footage does not identify those loads or their startup requirements.

Write down what needs to operate at once and what can be delayed or turned off. Do not assume a “whole-house” label means every appliance can run at the same time; transfer-switch and load-management configurations are model- and site-specific.

Running and starting demand are different

Running watts describe a load while it is operating. A motor-driven load may need a brief higher starting demand when it begins. The nameplate may provide voltage, amperage, full-load amps, or locked-rotor amps rather than a single starting-watt figure; those values must be interpreted for the actual system by a qualified professional.

A generator’s ability to start a motor is not established by adding every “starting watt” row in a general chart. The relevant question is what else is running when a specific motor starts, whether starts overlap, and how the generator’s transient/motor-starting capability performs at the fuel, voltage, and site conditions in the project.

A safe, source-based planning process

1. Choose the loads. Separate must-run equipment from loads that can be excluded, switched off, or sequenced.
2. Collect exact data. Photograph nameplates and manuals for compressors, pumps, electric heating/water heating, appliances, and any medical equipment. Use actual amperage and starting information where provided.
3. Build the coincident-load list. Record running loads expected to operate at the same time. Note when motors may start, and which loads could overlap.
4. Check the exact generator. Read the current model specification for the intended fuel, voltage, phase, standby rating, motor-starting capability, and transfer-switch configuration. Ratings may differ with fuel and site conditions.
5. Have a professional verify the complete system. The assessment should account for wiring/transfer equipment, load management, fuel supply and pressure, environmental derating, local requirements, and commissioning.

Worked example: what an approximate chart can—and cannot—show

The following is arithmetic using the upper ends of ranges in Champion’s published chart, checked October 4, 2026. It is not an example home measurement, not a recommendation, and not a generator-size calculation.

If the AC’s listed upper starting value occurred while all other example loads were running, the simple chart arithmetic is 5,870 − 3,500 + 6,000 = 8,370 W. If the pump and AC started at the same time, the same simplified arithmetic would be 9,370 W. If only the pump started, it would be 6,870 W.

This calculation exposes the importance of timing and load overlap; it does not verify motor acceleration, generator transient response, voltage drop, power factor, or the actual nameplate values. Do not purchase from this total. Provide the load list to a qualified local professional for a model-specific assessment.

Fuel and model ratings matter

Dual-fuel standby units may have different ratings on natural gas and LP. For example, the manufacturer’s current specification sheet for Generac model G0073270 lists 26 kW on LP and 24 kW on natural gas. That example describes one exact model; it is not a statement about all Generac units or a recommendation that this model fits a home.

Before comparing any ratings, match model number, fuel, phase, voltage, ambient/altitude requirements, and transfer-switch/load-management package. See our source-linked Generac and KOHLER model pages. For broader concepts, review generator ratings and derating.

Information to gather before a professional assessment

This is a blank static planning sheet. It records information; it does not add watts or produce a size.

Also note the fuel at the site, service/panel configuration, any medical-device requirements to discuss with a clinician or device supplier, and relevant property or permit constraints.

When professional review is essential

Have a qualified local professional assess the system before purchasing equipment or planning a connection. This is especially important for central HVAC, well/sump pumps, electric heat or water heat, multiple transfer panels, medical loads, high service amperage, load shedding, gas-line changes, or installations in constrained locations. The professional must apply the exact equipment instructions and local code/permit requirements.

Do not open an electrical panel, install a transfer switch/interlock, modify fuel piping, or use a calculator’s output as a substitute for the design. Standby Generator Guide has not performed an on-site load calculation and does not provide licensed electrical or fuel-system advice.

Continue with our guides to the installation process, project-cost scope, and generator safety.